Basal cell carcinoma. A red, raised ulcer-like growth called a basal cell carcinoma seen on a patient's face. Skin folds of the patient's eye are at top right. This cancerous tumour is also known as a rodent ulcer. The cells of the tumour are derived from the basal (innermost) skin layer. Basal cell carcinoma is thought to be caused by over-exposure to sunlight. Fair-skinned people over the age of 50 are most at risk, and it occurs commonly on the face. The cancer starts as a small flat nodule and grows slowly, then breaks down at the centre to form a shallow ulcer. It rarely spreads to other parts of the body. Treatment involves surgical removal or radiotherapy.